Concrete roof - support structure for wet mud

We are building (with contractor) a sub-grade garage with concrete roof to allow parking on top.
Right now we have completed walls and 4 I beams in place. The structure is 28' x 40', the 27' I beams sit at 8' centers. We need to frame up a floor in between the I beams / walls that can support 6" of wet mud.
By my calculations we can run 2x12x8' at 16 oc. The ends of the 2x will rest on the lower beam flanges and on a 4x6 beam on the wall ends supported by 4x4 @ 2' oc. Then 3/4" cdx for the deck.
Myself and the contractor rejected the engineers original plan due to extreme cost. The structure, once cured, meets the engineering requirement. I want to save the labor/ material mark up and build the floor structure myself. The floor will need to support 60 lbs/sqft (dead load) plus the workers walking around during pouring.
Deflection would need to be at a minimum. Whatever i come up with I'll send to the engineer, he wants to see it before we pour. Just looking for any thoughts or opinions. Thanks - Tom

Concrete roof - support structure for wet mud

tvanharp -

Make sure you have enough steel (small size and close together) to minimize the size of the cracking. All the sealing in the world will do little good if there are cracks. You will obviously have shrinkage cracks, but they must be minimized in spacing and size.

It is more than a structural problem and parking structures are unique creatures.

Concrete roof - support structure for wet mud

Quote:

Originally Posted by concretemasonry

You will obviously have shrinkage cracks, but they must be minimized in spacing and size.

There are shrinkage reducing additives you can specify when ordering your concrete. We used one called Eclipse on a high school job we did about two years ago. Getting the air entrainment to behave itself was a challenge, but once we got it dialed in, it worked pretty good.

Concrete roof - support structure for wet mud

We will be spanning 8' with 6" thick concrete not sure what the rebar specs are but i have a signed and approved engineering plan that cost me $2000. I do know on the plan it calls for cuts along the beam centers so it cracks on the beams. Also my contractor has done these types of structures before.

Concrete roof - support structure for wet mud

Flexicore is a precast concrete panel. I'm gonna guess 4-6' wide and maybe 50' would be its max length. They bring it in on a semi, set them with a crane, seal the joints and they are done!

Yes i did look into that. I used to live in wi. Out there it was called spancrete, i did find a company in spokane wa, that sold the same precast / pre-tensioned panels but the cost was about $11,000 just for the panels, and we're not sure if they could even get the trucks to our location. Not to mention the delivery and install cost, we are pretty remote with a half mile dirt road that winds up a mountain.

Concrete roof - support structure for wet mud

Flexicore panels are very often 24" wide and allow much smaller cranes and allow faster, cheaper and easier construction/placement on smaller jobs than the wider planks that are 40", 48" or 86" wide. Because Flexicore is a wet cast prestressed plank it has a much smoother bottom surface the the wider planks like Spancrete, SpanDeck and the others that use a much coarser, drier low slump concrete.

All precast planks will have to have a topping (2" or so) to provide a smooth top surface. The planks do eliminate any need for scaffolding, forming and shoring.